Headless computer entities are known in the art, and are also known as “headless appliances”. A known headless computer entity comprises a data-processor, memory, a plurality on input\output ports or the like, and an operating system. However, headless appliances are generally designed without user interfaces, and lack a keyboard, pointing device e.g. mouse or track ball, and visual display monitor. This has the advantages both of reducing the cost of ownership, since the cost of the user interface hardware need not be borne by the purchaser, and also inhibits interference with the operation of the appliance.
If an operating system failure occurs in some way, either due to data corruption a software bug or the like, then a headless appliance will simply stop functioning, and cannot be repaired manually except by replacing a data storage device, e.g. a system disk, with a new manufactured system disk containing a default operating system and default operating system configuration settings for the whole appliance. Replacement of the disk may involve erasing all data on the appliance. In a headless appliance there is no way that a human administrator of the appliance can directly manually install software updates to an internal operating system, or to application software since there is no system console.
A prior art solution used in headless appliances is to place an operating system in a flash ROM, thereby ensuring that the operating system cannot be easily damaged. This is fine for appliances in which an operating system can be contained in a flash ROM, such as a network attached storage device (NAS). However, where a headless computer entity is running applications, the operating system is far to large to fit into a flash ROM and has to be stored on a hard disk. Hard disks are vulnerable to corruption and damage and there is a risk of operating system failure where the operating system is stored on a hard disk.
A prior art solution for rectifying failure of an operating system stored on a hard disk includes a system developed by Microsoft® and Intel®, in which hardware and BIOS additions to a processor and hard disk ensure that if a running primary operating system fails in any way, for example fails to boot, locks up or crashes, then the BIOS switches to another identical copy of the operating system stored on a hard disk. This system is based on:                An NVRAM message passing interface between a BIOS and the primary operating system, allowing the primary operating system to indicate to the BIOS that it has booted successfully.        Hardware “watchdog” timers which reset the hardware if the primary operating system fails to clear the timers, so that if the operating system locks, the watchdog timers are not cleared, and the system automatically detects this and resets the BIOS extensions which trigger a boot from a second partition on the hard disk containing a secondary operating system if the primary operating system stored on the first partition fails to boot twice.        Software to check that the primary operating system fully boots, by monitoring key services provided by the operating system, and making sure that they are running smoothly after boot of the operating system.        
The concept of an operating system rebuild function in which a secondary operating system rebuilds a primary operating system from a compressed pristine version of the primary operating system is known in the Microsoft\Intel system. However, this known system assumes that the primary operating system is static and invariant with time.
In the case of computer entities running applications, there are many configuration settings which need to be made and reapplied. Therefore, in a computer entity running applications, there are other parts of software and code beyond the operating system which may need to be included in a full system rebuild following a failure of an operating system. The known rebuild system of Microsoft\Intel does not address the problem of how to rebuild a full system including applications settings in an automated manner, without the requirement for a user console.
The above known approach has the disadvantage and risk of failure if applied to a headless computer entity. In a headless computer entity, there is no way an administrator of the entity can manually install software updates to the operating system or application software. Therefore, the entity needs some mechanism to update the operating system, but when the entity operating system is on disk, it is very difficult for a running operating system to update itself reliably. Further, if a running operating system on a headless entity attempts to update itself, and a fault occurs during an update, then the headless entity will crash and stop working, unless there is an effective automatic operating system rebuild scheme in place, which also restores applications settings.
What is required is an install or rebuild process that is triggered in a variety of ways such that the process can be fully automated or manually controlled depending upon the type of trigger. Additionally, there is a requirement for a rebuild process that is sensitive to the status of the computer entity data, such that if corrupted data is detected as part of the rebuild process this data would be deleted and replaced with uncorrupted default data. Conversely, if the data is uncorrupted the rebuild process would not unnecessarily replace this data with default data.